There are many instances in which it is necessary to monitor average density, relative volumes, or properties related to them in a dispersed two-phase system. Processes for which the relative volumes or average density can be used for monitoring or control are especially of interest. Examples of applications include determining relative volumes in distillation in chemical processing, turbidity in environmental monitoring or chemical processing, or the presence or absence of two phases during heat transfer.
Visible light, gamma-rays, X-rays, and neutrons have been used to monitor density in flowing, two-phase fluids. The fact that two-phase fluids scatter light has been taken advantage of, and scattering or attenuation of the beam has been used to monitor the dispersed phase. For most applications, the accuracy obtained with these techniques has been sufficient. A major drawback has been that gamma-rays, X-rays, and neutrons create safety hazards. Though systems based on attenuation are cumbersome and complex, they have been available for many years and are still in use.
Because attenuation produces a logarithmic response, the range of operating conditions that can be monitored is great. However, accuracy and sensitivity are adversely affected by the relative amount of dispered phase and by the size distribution of the bubbles, droplets, or particles. Measurement errors of 100% frequently occur for relative volumes of gaseous phase as low as 20%. Such large errors are sometimes unacceptable, for example, in nuclear reactor applications.
A method and apparatus for determining the density of fluids is disclosed in U.S. Pat. No. 2,316,239 (Hare). Density is measured by directing a beam of radiation into the fluid. Attenuation or scattering of the radiation, or both, are used to determine its density. A method for determining the percent solids in a saturated solution by measuring the absorption of radiant energy is also disclosed in U.S. Pat. No. 3,368,389 (Barnet).
Laser beams have successfully been used in single-phase and two-phase fluids for the measurement of velocity. These velocity measurements are based on scattering of light by a dispered phase that is either present in the fluid or added to it. The frequency of bursts of scattered light indicates the velocity of the particles as they pass through alternating dark and light bands. Principles and Practice of Laser-Doppler Anemometry, F. Durst, A. Melling, J. H. Whitelaw, Academic Press, 1976.
Lasers and collimated light have also been used to measure density in single-phase systems using refraction, diffraction, or a combination of refraction and diffraction.
U.S. Pat. No. 4,381,895 (Hughes et al.) discloses a method and apparatus for determining the concentration of dissolved solids in a flowing solution. A collimated monochromatic light beam is passed through the sample to a light sensor to measure refraction in a single phase. The method described by Hughes could not be used to accurately or sensitively measure average density or relative proportions in a dispersed two-phase system because it does not include a means for causing the entire cross-section of the beam to be refracted by the dispersed phase. If a dispersed phase were present, a portion of the beam would be scattered or attenuated, and the refractivity that would be measured would be that of the continuous phase.
Japan Patent No. 201,236 (Yoshino) discloses the use of refraction of a laser beam in a liquid to monitor refractivity. The output is an electric signal that can be used in a on-line system. The method described by Yoshino could not be used to accurately or sensitively measure average density or relative proportions in a dispersed two-phase system because it does not include a means for making the beam fine enough to be refracted rather than scattered by the dispersed phase.
Using interference patterns greatly increases sensitivity of the measurement of refraction. In Rayleigh's refractometer, a monochromatic light beam is split and passed through different solutions in matched cells. A lens is used to bring the split beams together, and the amount of compensation needed to bring them into phase, as indicated by interference, is used to measure slight differences in refractive index. (Jenkins et al., Fundamentals of Optics, pp. 258-259, third edition, McGraw-Hill, New York, 1957. The method described by Jenkins could not be used to accurately or sensitively measure average density or relative proportions in a dispersed two-phase system because it does not include a means for making the beam fine enough to be refracted rather than scattered by the dispersed phase.
Diffraction was used in combination with refraction by A. B. Nafarrate, "Diffraction Refractometer," IBM Technical Disclosure Bulletin, 13 (1), 258 (1980). A transmission diffraction grating was placed in a wall of a cell containing a medium. The diffraction pattern for a vacuum or for air was used as a reference. Measurements were made of the distance between the zero order beam and the first order beam. The amount this distance changed allowed the refractive index of the test substance to be determined. Changes in interference patterns is a more sensitive method than simply measuring refraction of a beam. The method described by Naffarate could not be used to accurately or sensitively measure average density or relative proportions in a dispersed two-phase system because it does not include a means for making the beam fine enough to be refracted rather than scattered by the dispersed phase.
The prior art lacks a method for easily, safely, and sensitively monitoring the relative volumes or average density of a transparent, dispersed two-phase fluid.